Alkylated diarylamines, such as alkylated diphenylamine, are well known in the art and function as stabilizers or antioxidants in a wide variety of organic materials, including, among other organic materials, mineral oil derived lubricants and synthetic lubricants. In this application a light colored liquid product (at about 20° C.) having a low concentration of unreacted diphenylamine is desirable for a number of practical reasons.
Alkylation of diarylamines, such as diphenylamine, with olefins in the presence of suitable alkylation catalysts is well known in the art. Typically unreacted diphenylamine when present in objectionable amounts has been removed by high temperature, high vacuum distillation of the crude product. These distillation techniques, made necessary by the high boiling points and thermolabile properties of the products are expensive process steps and lead to further product loss. Other techniques have used to reduce the amount of unreacted diphenylamine; for example methods have directed to using extremely large excess of alkylate in comparison to the diphenylamine, U.S. Pat. No. 5,186,852 (Ishida et al.) exemplifies eight fold excess of alkylate. As is apparent, the use of a large excess of alkylate is not particularly cost effective. Recycling of alkylate has been problematic; the residual alkylating agent has lower reactivity than the starting alkylate partly due to rearrangement products being formed and the alkylation agent can be halogenated by the catalyst in Friedel-Crafts alkylation. Commonly, the unreacted alkylate has been used for other applications such as fuels, however depending upon the halogen content these alternative uses may be limited. U.S. Pat. No. 3,452,056 describes the preparation of a mixture of 80% dinonyldiphenylamine, 15% mono-nonyldiphenylamine and about 2% unreacted diphenylamine using Friedel-Crafts alkylation in the presence of Lewis Acid catalysts wherein AlCl3 and ZnCl2 are mentioned as suitable catalysts. Lewis Acid catalysts are somewhat undesirable, since they may lead to halogenation of the product and due to degree of process water generated in the purification.
In a different approach, commonly employed for preparing isobutylene derived diphenylamines, a more reactive scavenging alkylate is used to further alkylate the unreacted diphenylamine. U.S. Pat. No. 2,943,112 (Popoff et al.) teaches a two step process whereby alkylation of diphenylamine with relatively unreactive olefins, such as secondary alkenes (column 4, line 9-23), is followed by an alkylation reaction with more reactive olefins to scavenge the unreacted diphenylamine. Popoff also teaches the use of acid activated clay as an alkylation reaction catalyst to achieve the desired light color.
Similarly, Franklin, U.S. Pat. No. 4,824,601, (column 1, lines 26-67), teaches the use of acidic clay catalysts to alkylate diphenylamine and further teaches that a light colored, liquid product may be prepared by process comprising reacting the alkylation reactants within certain molar ratios and temperature ranges for a time sufficient to ensure the alkylated product contains less than 25% dialkylated diphenylamine. This low proportion of dialkylated diphenylamine is disclosed as necessary to avoid the formation of crystallized, solid products, which are not advantageous in terms of ease of handling, transportation, storage and incorporation into the substrate to be stabilized. This crystallization issue is a concern when alkylating diphenylamine with diisobutylene and is a lesser concern when employing nonene.
Also in addressing the same problem, namely, preparing an effective antioxidant from diphenylamine and diisobutylene that is liquid at room temperature, Lai in U.S. Pat. Nos. 5,672,752 and 5,750,787, teaches processes for alkylating diphenylamine with linear alpha olefins and diisobutylene in the presence of a clay catalyst. These processes as disclosed, selectively result in a higher proportion of monoalkylated diphenylamine and a lower proportion of unsubstituted diphenylamine and/or disubstituted or polysubstituted diphenylamines. These patents further disclose that to obtain the desired liquid product, the ratio of olefin to diphenylamine in the reaction mixture, together with reaction temperature and time is important to give a product mixture with less than 25% dioctyldiphenylamine, less than 25% unreacted diphenylamine and greater than 50% by weight monooctyldiphenylamine based on the total weight of the diphenylamine and alkylated DPA.
In U.S. Pat. No. 6,204,412 (Lai) Lai discloses yet another method of alkylating diphenylamine to obtain a light colored, liquid product, which comprises a two step method wherein, in the second step, a second olefin is added to the reaction mixture containing diphenylamine and diisobutylene (and/or an alpha-olefin of the disclosed formula) to scavenge or reduce the amount of unreacted diphenylamine in the product As with U.S. Pat. Nos. 5,672,752 and 5,750,787, specific mole ratio ranges, reaction temperatures and reaction times are disclosed as important to obtain the desired alkylated diphenylamine that is liquid at room temperature.
U.S. Pat. No. 6,315,925 (Aebli et al.) discloses alkylating diphenylamine with excess nonene in the presence of an acid earth and in the absence of a free protonic acid. The particular composition is defined by an area percent of a gas chromatograph, more particularly defined by having not more than 3.5% by area trinonyldiphenylamine.